1. Field of the Invention
The subject invention is in the fields of stringed instruments and composite material and structures. More specifically, it is in the field of that part of the structure of stringed instruments to which the strings are attached in order to provide, resist and maintain the tension forces in the strings. Still more specifically, the invention relates to the structural part in a piano which performs these functions, commonly known as the plate or plate assembly.
2. Discussion of the Prior Art
It is clearly evident from discussions with people skilled in the art and from study of pertinent literature and prior art patents that the design of pianos has evolved over many years. It is also evident that understanding of and theories and explanations about how the various elements and components of pianos function and cooperate in producing the music are varied. The qualities of the music are said to be a function of the interactions among the strings, the plate, the sounding board, the frame, the mechanisms for striking the strings and such detailed things as the degrees of stiffness and resilience in the pads of the hammers which strike the strings. There is general agreement, however, that the characteristics of the plate, the structure to which the strings are attached, have a fundamental influence on the quality and consistency of the music. Further, two of the more important characteristics are (1) structural stability in terms of dimensions and shape and (2) structural damping. The stability is needed in order for the piano to stay in tune, regardless of the effects of humidity, temperature, time and handling. The damping, according to well accepted theory and empirical determinations, is needed so that there will be minimal resonant vibrations in the structure since such resonances produce undesired sound and/or otherwise interfere with the production of sound by the strings.
Prior art plates are made of grey cast iron. With this material it is necessary, for all but the smallest, to use a rim type construction to limit flexing to acceptable amounts. Such construction is difficult and expensive and results in heavy, cumbersome plates. Furthermore, grey cast iron has low internal damping (i.e. it is elastically efficient) and tends to resonate in response to string vibration, particularly in the treble frequency range.
Another factor which adds to the cost of cast iron plates is that they are not amenable to mass production techniques, i.e. the castings must age and considerable, special hand work is required on each plate and its installation. This factor is largely attributable to the internal stresses inherent in iron castings. Because of these stresses castings have a strong tendency to change shape when the casting is machined or holes are drilled in it. Also, the shape changes as the stresses relieve with time. It is difficult to achieve uniformity in each casting and among castings.
In addition to the use of rim type construction to limit flexing, cast iron plates are often structurally integrated with a surrounding wooden structure known as a case. To effectively help limit flexing of the plate, the case itself is heavy and cumbersome and compounds the problems associated with cost and weight.
Examples of attempts to solve the problems of undesired resonance in the plate and excessive weight and cost can be found in the prior art. For example, U.S. Pat. No. 3,144,800 describes a pianoforte comprising a moulding and a wooden assembly. The moulding combines into one unit the frame (another term for plate) and the top, bottom and sides of the case. The wooden assembly is the back of the case and provides the resonance needed for proper operation of the instrument.
The moulding may also incorporate an element known as the key bottom and one or more bracing struts. It is pointed out that certain parts which are particularly highly stressed may be formed by winding glass fibre on a mandrel and impregnating the winding with resin. The mandrel may be metal or made of the resin treated glass cloth used in the rest of the plastic unit and remains as part of the unit.
Instruments made according to this patent have not become commercially successful and accordingly can be recognized as not providing a good solution to the problems described. This lack of success can be attributed to factors including the amount of hand work required in manufacturing the moulded unit, the difficulty of achieving uniformity among units even with the best affordable manufacturing efforts, the lack of applicability of the technology to a variety of types of pianos, and the use of glass fibre as the reinforcing material.
U.S. Pat. No. 3,562,027 addresses the problem of reduction of weight of the plate by design details and selection of materials (cast irons). It is claimed that cast iron having a tensile strength of at least 37 kg/mm.sup.2 and an arbitrarily defined loss factor between certain limits can be used to produce lighter weight cast iron plates without sacrificing tonal qualities. The patent teaches the material technology, but not the design features made possible by the materials. The amount of weight saved is not indicated. Further, the cost of a plate made according to this patent would be higher than the cost of equivalent current state-of-the-art cast iron plates.
U.S. Pat. No. 3,564,963 illustrates the need for minimizing the effects of vibrations of the plates on the tonal qualities of the piano. This patent covers means for " - - - damping out the undesirable ringing modes of shock-excited metal structures - - - ". The damping means comprise metal masses positioned to contact the vibrating braces of the plate, with the metal masses cushioned in dead rubber. Such means are an add-on feature and accordingly add to the cost of the piano. Also the means must be specially designed and properly adjusted to suit each type of piano and each piano of any type. This adds further to the cost and the effectiveness is vulnerable to the correctness of the adjustment and any effects temperature changes and time may have on the adjustments.
U.S. Pat. No. 4,144,792 reflects a different school of thought about how pianos function. In this case the theory is that the vibrations of the plate improve the operation of the instrument and the patent relates to improved means for casting the plate and to shaping of its surfaces to improve its vibration characteristics and aesthetics. The teaching of other patents and of experience in the field raise serious doubts about the validity of the theoretical approach. However, what constitutes good operation of the instrument is a matter of subjective judgment. High quality operation can be achieved when the vibrations of the plate, for whatever reason, are involved to as little extent as possible in the operation of the instrument. Minimizing or eliminating effects of plate vibration on instrument operation is much more readily accomplished than attempting to achieve, with good consistency, particular vibratory cooperation among the strings and sections of the plate.
U.S. Pat. No. 4,145,948 relates to a stringed musical instrument and particularly to the structural element which supports the strings and which reacts the forces generated by the tension of the strings. The structural element, the neck of a guitar for example, is made of graphite fiber reinforced plastic material. This material replaces wood and is described as providing a neck which is lighter but stiffer than the equivalent wooden neck as well as being more stable dimensionally under the influences of temperature and humidity variations. This dimensional stability, in combination with the minimal vibrational resonance of the neck, provide improved and stable acoustic tonal qualities. It is obvious that the benefits of using graphite fiber reinforced plastic material in a guitar could be realized by using such material in other stringed instruments, such as pianos. Other advantages which result from doing so, as described herein, are not obvious, particularly if the composite material is used to replace cast iron instead of wood. The other benefits include, as stated elsewhere, reduced weight and cost, neither of which would be particularly relevant to a guitar or similar small instrument.
Other prior art, related but of more general interest, includes the following U.S. Patents:
U.S. Pat. No. 3,656,395 PA1 U.S. Pat. No. 3,805,663 PA1 U.S. Pat. No. 3,943,816 PA1 U.S. Pat. No. 4,084,476 PA1 U.S. Pat. No. 4,119,009 PA1 U.S. Pat. No. 4,121,492 PA1 U.S. Pat. No. 4,144,793 PA1 U.S. Pat. No. 4,161,130 PA1 U.S. Pat. No. 4,200,023 PA1 U.S. Pat. No. 4,131,362